Intrinsic foot muscles act to stabilise the foot when greater fluctuations in centre of pressure movement result from increased postural balance challenge

© 2020 The Author(s) Background: Increased postural balance challenge is associated with more fluctuations in centre of pressure movement, indicating increased interference from the postural control system. The role of intrinsic foot muscles in balance control is relatively understudied and whether such control system interference occurs at the level of these muscles is unknown. Research Question: Do fewer fluctuations in intrinsic foot muscle excitation occur in response to increased postural balance challenge? Methods: Surface EMGs were recorded using a grid of 13 × 5 channels from the plantar surface of the foot of 17 participants, who completed three balance tasks: bipedal stance; single leg stance and bipedal tip-toe. Centre of pressure (CoP) movement was calculated from simultaneously recorded force plate signals. Fluctuations in CoP and EMGs for each task were quantified using a sample entropy based metric, Entropy Halflife (EnHL). Longer EnHL indicates fewer signal fluctuations. Results: The shortest EMG EnHL, 9.27 ± 3.34 ms (median ± interquartile range), occurred during bipedal stance and the longest during bipedal tip-toe 15.46 ± 11.16 ms, with 18.80 ± 8.00 ms recorded for single leg stance. Differences were statistically significant between bipedal stance and both bipedal tip-toe (p < 0.001) and single leg stance (p < 0.001). CoP EnHL for both anterior-posterior and medial-lateral movements also differed significantly between tasks (p < 0.001, both cases). However, anterior-posterior CoP EnHL was longest for bipedal stance 259.84±230.22 ms and shortest for bipedal tip-toe 146.25±73.35 ms. Medial-lateral CoP EnHL was also longest during bipedal stance 215.73±187.58 ms, but shortest for single leg stance 113.48±83.01 ms. Significance: Fewer fluctuations in intrinsic foot muscle excitation occur in response to increased postural balance challenge. Fluctuations in CoP movement during balance must be predominantly driven by excitation of muscles extrinsic to the foot. Intrinsic foot muscles therefore likely play a greater role in stabilisation of the foot than balance control during the postural tasks studied

Comparison between surface electrodes and ultrasound monitoring to measure TMS evoked muscle contraction

INTRODUCTION: Transcranial magnetic stimulation (TMS) is widely employed to explore cortical physiology in health and disease. Surface electromyography (sEMG) is appropriate for superficial muscles, but cannot be applied easily to less accessible muscles. Muscle ultrasound (mUS) may provide an elegant solution to this problem, but fundamental questions remain. We explore the relationship between TMS evoked muscle potentials and TMS evoked muscle contractions measured with mUS. METHODS: In 10 participants we performed a TMS recruitment curve, simultaneously measuring motor evoked potentials (MEPs) and mUS in biceps (BI), first dorsal interosseous (FDI), tibialis anterior (TA) and the tongue (TO). RESULTS: Resting motor threshold (RMT) measurements and recruitment curves were found to be consistent across sEMG and mUS. DISCUSSION: This work supports the use of TMS-US to study less accessible muscles. The implications are broad but could include the study of a new range of muscles in disorders such as amyotrophic lateral sclerosis

Electrodes' Configuration Influences the Agreement between Surface EMG and B-Mode Ultrasound Detection of Motor Unit Fasciculation

Muscle fasciculations, resulting from the spontaneous activation of motor neurons, may be associated with neurological disorders, and are often assessed with intramuscular electromyography (EMG). Recently, however, both ultrasound (US) imaging and multichannel surface EMG have been shown to be more sensitive to fasciculations. In this study we combined these two techniques to compare their detection sensitivity to fasciculations occurring in different muscle regions and to investigate the effect of EMG electrodes' configuration on their agreement. Monopolar surface EMGs were collected from medial gastrocnemius and soleus with an array of 32 electrodes (10 mm Inter-Electrode Distance, IED) simultaneously with b-mode US images detected alongside either proximal, central or distal electrodes groups. Fasciculation potentials (FP) were identified from single differential EMGs with 10 mm (SD1), 20 mm (SD2) and 30 mm (SD3) IEDs, and fasciculation events (FE) from US image sequences. The number, location, and size of FEs and FPs in 10 healthy participants were analyzed. Overall, the two techniques showed similar sensitivities to muscle fasciculations. US was equally sensitive to FE occurring in the proximal and distal calf regions, while the number of FP revealed by EMG increased significantly with the IED and was larger distally, where the depth of FE decreased. The agreement between the two techniques was relatively low, with a percentage of fasciculation classified as common ranging from 22% for the smallest IED to 68% for the largest IED. The relevant number of events uniquely detected by the two techniques is discussed in terms of different spatial sensitivities of EMG and US, which suggest that a combination of US-EMG is likely to maximise the sensitivity to muscle fasciculations

Short report presenting preliminary evidence of impaired corticomuscular coherence in an individual with Developmental Coordination Disorder

Background It has been suggested that developmental coordination disorder (DCD) could be caused by a ‘dysconnection’ in brain and skeletal muscle communication. To date no previous work has examined the integrity of this neuromuscular process in individuals with DCD. Aims To conduct a feasibility study for measuring functional connectivity of the brain and muscle in an individual with DCD using corticomuscular coherence (CMC). Methods and Procedures An individual with DCD and a typically developing (TD) participant completed a series of sustained 5-second voluntary isometric hand contractions (15 ± 5 % MVC) on a handheld dynamometer under both single and dual task (i.e., counting backwards) conditions. EEG, EMG and force data were collected. Outcomes and Results The participant with DCD displayed poorer force steadiness and higher mental demand compared to the TD participant and in dual task conditions. The TD participant displayed a commonly observed pattern of CMC that was highly localised over the contralateral hand area, the DCD participant displayed a less localised CMC across cortical regions. Conclusions and Implications These findings support the feasibility of measuring CMC in DCD populations and offer some, albeit preliminary, evidence of impaired communication between the brain and muscles in these individuals

An evaluation of common markers of muscle denervation in denervated young-adult and old rat gastrocnemius muscle.

A large part of age-related muscle wasting is due to incomplete reinnervation of fibres that have become denervated following motoneuron loss. Neural cell adhesion molecule (NCAM) and sodium channel NaV1.5 are considered markers for denervation, but the time course of changes in their expression following denervation has never been systematically evaluated in young-adult and old muscle. To assess the time course of denervation-induced changes in their expression, the left gastrocnemius muscle in 15 young-adult (5-month) and 10 old (25-month) male Wistar rats was denervated for 1, 2 or 4 weeks, while the right muscle served as an internal control. Sections were stained for α-bungarotoxin, to visualise the neuromuscular junctions, combined with NCAM, polysialylated NCAM (PSA-NCAM) or NaV1.5. In young-adult animals, denervation induced a transient decrease in junctional and cytoplasmic NCAM expression, while in the old NCAM expression was increased after 2 weeks. Cytoplasmic PSA-NCAM was increased in both young-adult and old fibres after 2 weeks denervation with a further increase after 4 weeks in the young only. The junctional PSA-NCAM was transiently increased or decreased in the young and old muscles, respectively. NaV1.5 expression decreased after 1 and 2 weeks of denervation in NaV1.5 in young muscle fibres before returning to control levels, whereas old muscle fibres displayed a transient increase after 1 week followed by a decrease and a return to control levels after 2 and 4 weeks respectively. In conclusion, NCAM and NaV1.5 are not unequivocally elevated with denervation and consequently are not adequate markers of fibre denervation

Surface electromyography can quantify temporal and spatial patterns of activation of intrinsic human foot muscles

Intrinsic foot muscles (IFM) are a crucial component within the human foot. Investigating their functioning can help understand healthy and pathological behaviour of foot and ankle, fundamental for everyday activities. Recording muscle activation from IFM has been attempted with invasive techniques, mainly investigating single muscles. Here we present a novel methodology, to investigate the feasibility of recording physiological surface EMG (sEMG) non-invasively and quantify patterns of activation across the whole plantar region of the foot. sEMG were recorded with a 13 × 5 array from the sole of the foot (n = 25) during two-foot stance, two-foot tiptoe and anterior/posterior sways. Physiological features of sEMG were analysed. During anterior/posterior epochs within the sway task, sEMG patterns were analysed in terms of signal amplitude (intensity) and structure (Sample Entropy) distribution, by evaluating the centre of gravity (CoG) of each topographical map. Results suggest signals are physiological and not affected by loading. Both amplitude and sample entropy CoG coordinates were grouped in one region and overlapped, suggesting that the region with highest amplitude corresponds with the most predictable signal. Therefore, both spatial and temporal features of IFM activation may be recorded non-invasively, providing opportunity for more detailed investigation of IFM function in healthy and patient populations

A micromechanical muscle model for determining the impact of motor unit fiber clustering on force transmission in aging skeletal muscle

© 2019, The Author(s). This study used a micromechanical finite element muscle model to investigate the effects of the redistribution of spatial activation patterns in young and old muscle. The geometry consisted of a bundle of 19 active muscle fibers encased in endomysium sheets, surrounded by passive tissue to model a fascicle. Force was induced by activating combinations of the 19 active muscle fibers. The spacial clustering of muscle fibers modeled in this study showed unbalanced strains suggesting tissue damage at higher strain levels may occur during higher levels of activation and/or during dynamic conditions. These patterns of motor unit remodeling are one of the consequences of motor unit loss and reinnervation associated with aging. The results did not reveal evident quantitative changes in force transmission between old and young adults, but the patterns of stress and strain distribution were affected, suggesting an uneven distribution of the forces may occur within the fascicle that could provide a mechanism for muscle injury in older muscle

Achilles tendon moment arm in humans is not affected by inversion/eversion of the foot: a short report

The triceps surae primarily acts as plantarflexor of the ankle joint. However, the group also causes inversion and eversion at the subtalar joint. Despite this, the Achilles tendon moment arm is generally measured without considering the potential influence of inversion/eversion of the foot during plantarflexion. This study investigated the effect of foot inversion and eversion on the plantarflexion Achilles tendon moment arm. Achilles tendon moment arms were determined using the centre-of-rotation method in MR images of the left ankle of 11 participants. The foot was positioned at 15° dorsiflexion, 0° or 15° plantarflexion using a Styrofoam wedge. In each of these positions the foot was either 10° inverted, neutral or 10° everted using an additional Styrofoam wedge. Achilles tendon moment arm in neutral foot position was 47.93 ± 4.54 mm and did not differ significantly when the foot was positioned in 10° inversion and 10° eversion. Hence, inversion/eversion position of the foot may not considerably affect the length of the Achilles tendon moment arm. This information could be useful in musculoskeletal models of the human lower leg and foot and when estimating Achilles tendon forces during plantarflexion with the foot positioned in inversion or eversion